At first I LOL-ed (L-ed-OL?). But now I’m thinking that it really is a pretty good question.
The Gemara tells us some classic cases of Geneivas Da’as. They all share a common denominator – they cause someone to be grateful for something you really didn’t do for him. One example: Someone comes to your house and he sees you run to the wine cellar and come up with a rare, expensive bottle of wine and open it up and pour him a glass. He thinks you opened it in his honor, and he is grateful and feels honored. What he doesn’t know is that you open up a bottle like this every week. “Taking” his gratefulness is therefore Geneivas Da’as.
From here it would seem that “recycling” shalach manos is a problem, or as marbehshalom put it – unethical.
However, the Gemara also says that when the person receiving the honor is simply fooling himself into believing it, then it’s not your problem. So, let’s say it was a normal bottle of wine, and it’s pretty common for someone to open a normal bottle of wine on Friday night, then you don’t have to worry about the fact that this guy has a big head and will think that you are doing it special for him, because as the Gemara says, “he is tricking himself.”
I think that it is fairly common for people to recycle gifts, but in a certain context. You might recycle to a regular friend an extra special shalach manos that you got from someone else, but you probably wouldn’t recycle shalach manos for a teacher you are especially close and grateful to. It really makes a difference who you are giving to.
My official CR p’sak: If it is someone to whom people would consider it normal to recycle, go for it. Just make sure they won’t find out (because that would be awkward). But for someone like a teacher or a really close friend, someone whom if you would tell people you recycled shalach manos to him/her, they would give you a look, to such a person it is quite possibly geneivas da’as to recycle to, and you shouldn’t. And it would be nerdy too.